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<a name="File-format-of-unformatted-sequential-files"></a>
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<hr>
<a name="File-format-of-unformatted-sequential-files-1"></a>
<h3 class="section">4.9 File format of unformatted sequential files</h3>
<a name="index-file_002c-unformatted-sequential"></a>
<a name="index-unformatted-sequential"></a>
<a name="index-sequential_002c-unformatted"></a>
<a name="index-record-marker"></a>
<a name="index-subrecord"></a>

<p>Unformatted sequential files are stored as logical records using
record markers.  Each logical record consists of one of more
subrecords.
</p>
<p>Each subrecord consists of a leading record marker, the data written
by the user program, and a trailing record marker.  The record markers
are four-byte integers by default, and eight-byte integers if the
<samp>-fmax-subrecord-length=8</samp> option (which exists for backwards
compability only) is in effect.
</p>
<p>The representation of the record markers is that of unformatted files
given with the <samp>-fconvert</samp> option, the <a href="CONVERT-specifier.html#CONVERT-specifier">CONVERT specifier</a>
in an open statement or the <a href="GFORTRAN_005fCONVERT_005fUNIT.html#GFORTRAN_005fCONVERT_005fUNIT">GFORTRAN_CONVERT_UNIT</a> environment
variable.
</p>
<p>The maximum number of bytes of user data in a subrecord is 2147483639
(2 GiB - 9) for a four-byte record marker.  This limit can be lowered
with the <samp>-fmax-subrecord-length</samp> option, although this is
rarely useful. If the length of a logical record exceeds this limit,
the data is distributed among several subrecords.
</p>
<p>The absolute of the number stored in the record markers is the number
of bytes of user data in the corresponding subrecord.  If the leading
record marker of a subrecord contains a negative number, another
subrecord follows the current one.  If the trailing record marker
contains a negative number, then there is a preceding subrecord.
</p>
<p>In the most simple case, with only one subrecord per logical record,
both record markers contain the number of bytes of user data in the
record.
</p>
<p>The format for unformatted sequential data can be duplicated using
unformatted stream, as shown in the example program for an unformatted
record containing a single subrecord:
</p>
<div class="smallexample">
<pre class="smallexample">program main
  use iso_fortran_env, only: int32
  implicit none
  integer(int32) :: i
  real, dimension(10) :: a, b
  call random_number(a)
  open (10,file='test.dat',form='unformatted',access='stream')
  inquire (iolength=i) a
  write (10) i, a, i
  close (10)
  open (10,file='test.dat',form='unformatted')
  read (10) b
  if (all (a == b)) print *,'success!'
end program main
</pre></div>

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